As semiconductor geometries continue to shrink, manufacturers have increasingly turned to optical techniques to perform non-destructive inspection and analysis of semiconductor wafers. Techniques of this type, known generally as optical metrology, operate by illuminating a sample with an incident field (typically referred to as a probe beam) and then detecting and analyzing the reflected energy. Ellipsometry and reflectometry are two examples of commonly used optical techniques. For the specific case of ellipsometry, changes in the polarization state of the probe beam are analyzed. Reflectometry is similar, except that changes in intensity are analyzed. Scatterometry is a specific type of optical metrology that is used when the structural geometry of a sample creates diffraction (optical scattering) of the probe beam. Scatterometry systems analyze diffraction to deduce details of the structures that cause the diffraction to occur.
Various optical techniques have been used to perform optical scatterometry. These include broadband spectroscopy (U.S. Pat. Nos. 5,607,800; 5,867,276 and 5,963,329), spectral ellipsometry (U.S. Pat. No. 5,739,909) single-wavelength optical scattering (U.S. Pat. No. 5,889,593), and spectral and single-wavelength beam profile reflectance and beam profile ellipsometry (U.S. Pat. No. 6,429,943). Scatterometry, in these cases generally refers to optical response information in the form of diffraction orders produced by periodic structures (e.g., gratings on a wafer). In addition it may be possible to employ any of these measurement technologies, e.g., single-wavelength laser BPR or BPE, to obtain critical dimension (CD) measurements on non-periodic structures, such as isolated lines or isolated vias and mesas. The above cited patents and patent applications, along with PCT Application WO03/009063, US Application 2002/0158193, US Application 2003/0147086, US Application 2001/0051856 A1, PCT Application WO 01/55669 and PCT Application WO 01/97280 are all incorporated herein by reference.
Most metrology techniques (including those just described) may be performed using monochromatic or polychromatic light. In the case where polychromatic light is used, the interaction between the probe beam and the subject is analyzed as a function of wavelength. In many cases, this increases the accuracy of the analysis. As shown in FIG. 1A, a representative implementation of an ellipsometer or reflectometer configured to perform this type of polychromatic analysis includes a broadband light source. The light source creates a polychromatic probe beam that is focused by one or more lenses on a subject. The subject reflects the probe beam. The reflected probe beam passes through an aperture and another series of one or more lenses to a detector. A processor analyzes the measurements made by the detector.
As shown in FIG. 1B, the broadband light source is a combination of two different sources: a visible light source and a UV source. The visible light source is typically a tungsten lamp and the UV source is typically a deuterium lamp. The outputs of the two lamps are combined using a beam combiner. Prior art beam combiners are usually formed by depositing a very thin partially transparent metal film, such as aluminum on a substrate. The surface of the film is coated with a protective layer of silicon dioxide or magnesium fluoride. A notable example of a UV to visible beam combiner is a 50/50 beam splitter. The output of the beam combiner is the probe beam produced by the broadband light source. The combination of the two separate lamps increases the spectrum of the probe beam beyond what would be practical using a single source.
Unfortunately, the use of broadband light sources of the type shown in FIG. 1 has known drawbacks. As shown in FIG. 1C, the resulting probe beam is not constant as a function of wavelength. Instead, there is a tendency for illumination levels to be weak for wavelengths that fall between the outputs of the two separate lamps. For this reason, broadband ellipsometers and reflectometers are typically configured to electronically compensate for the uneven nature of their probe beams. For many applications this produces acceptable results. This is not the case for the low illumination levels required by advanced systems. In these cases, the electronic compensation process actually masks data that would otherwise reach the detector for analysis. This degrades detector performance and reduces overall sensitivity.
For these reasons and others, a need exists for improved devices for creating probe beams having uniform illumination levels across a range of useful wavelengths. This need is especially important for metrology tools that require the combination of multiple illumination sources to create wide spectrum polychromatic probe beams.